Yes, you can still find the option to install apps from unknown sources on the Fire TV Stick 4K Max in developer mode. Although Amazon has hidden this menu in recent updates, it's still accessible by following the appropriate steps. To access it, you first need to enable developer mode by going to Settings > My Fire TV > About, then press the centre button on your remote 7 times. Once developer mode has been activated, you'll see a new menu called 'Developer Options'. It's in this menu that you'll find the 'Applications from unknown sources' option, which you can activate to install external APKs.